The invention relates to a radio frequency (RF) system whose controlled electromagnetic fields are monitored by electronic devices which indicate any disturbances to the fields caused by an unauthorized intrusion. There is thus provided a means of protecting high value isolated resources from unwanted physical presence therein. The resources to be protected range from physical structures such as buildings, vehicles and planes to zones of space (temporary airfields and storage areas).
The prior art for protection against intrusion isolated resources had limitations as to the extent of the discrimination provided. The present invention uses VHF frequencies to obtain a strong perturbation from human intruders. It also makes possible discrimination against nuisance alarms from small animals and birds. It further eliminates the loss of sensitivity due to changing environmental conditions. Finally, the use of two concentric cables restricts the protected zone to preselected shapes such as torroidal.